Enabled the Custom Block plugin (in the generic area).
Created a block called Directors, saved.
Went to the Block plugins area to edit the new Directors (Custom Block), added names, saved.
Block was already on the right sidebar, I went to Setup 5.6 and move the Directors block from the right sidebar, to the middle, and then to the left sidebar, saved.
The Directors block appeared as expected on the left sidebar.
Went back to the Custom Block plugin (in the generic area) to create another block called Sponsors, saved.
Noted that there were now two Custom Blocks listed.
Went to the Block plugins area. Noticed the new Sponsors (Custom Block) item. The Directors (Custom Block) had disappeared. Checked home page, sure enough, Directors no longer visible.
Went to Setup 5.6, Directors block nowhere to be seen, but new Sponsors block now in the right sidebar area.
Directors block vanished, as if it were never there. Still listed under Custom Block in the generic area.

The custom blocks appear to be "remembered".
Followed similar steps and reproduced bug.
However, when I switched the labels of the blocks with one another in the custom block plug-in, my first block block reappeared with all it's content on the right sidebar. Also the state of the custom block (enabled/disabled) is also remembered.
Also, whenever entering Step 5.6, observed the custom block defaulting to the right sidebar.